The present invention relates to systems and methods for stretching polymer films. More particularly, it relates to in-line devices provided with the stretching system for handling or taking-away film following a stretching operation.
There are a variety of reasons to stretch polymer films. Stretching can enhance or generate desired mechanical, optical, and other film properties. For example, polymer films can be stretched to provide a desired degree of uniaxial or near uniaxial orientation to provide useful optical properties. In general, perfect uniaxial orientation of a birefringent polymer results in a film (or layers of a film) in which the index of refraction in two of three orthogonal directions is the same (for example, the width and thickness of a film). The index of refraction in the third direction (for example, along the length (L) direction of the film) is different from the indices of refraction in the other two directions. Typically, perfect uniaxial orientation is not required and some degree of deviation from the optimal conditions can be allowed depending on a variety of factors including the end-use application of the polymer film.
Regardless of whether uniaxial orientation in the stretched film is achieved or desired, systems for stretching the film in at least one direction typically includes a stretching station or device, and one or more stations or devices downstream of the stretching device that handle and optionally further process the stretched film. The film stretching device can assume various forms. Some conventional film stretching devices are, or are akin to, a tenter, and generally entail gripping opposing edges of the film with tenter clips (or other gripping device). The tenter clips are connected to tenter chains that ride along diverging tracks or rails. This arrangement propels the film forward in a machine direction of film travel and stretches the film in a transverse direction. Conventional tenter-type film stretching devices may not achieve uniaxial stretch. Other film stretching devices have been developed, described for example in U.S. Pat. Nos. 6,916,440; 6,936,209; and 6,939,499 that beneficially achieve uniaxial or substantially uniaxial stretch.
While extensive efforts have been made to develop optimized film stretching devices, the film handling stations or devices downstream of the film stretching device have generally remained unchanged. The downstream station is oftentimes referred to as a “take-away” device or station, and transports film received from the stretching device. The take-away device includes a conveyor-type system for transporting the film, conventionally provided as one or more gripping mechanisms carried by a chain or belt. The gripping mechanisms interface with opposing surfaces of the film, normally at or near the film's edges. The gripping mechanisms can include tenter clips carried by an endless belt and arranged to grip an edge of the film. Other gripping formats include opposing conveyors arranged to interface with (e.g., grip) the opposing major surfaces of the film. The set of opposing conveyors are normally located to contact the film at or near an edge of the film, and can include a series of discrete pads carried by an endless chain (e.g., each pad is attached to an individual link of the chain). The pads each provide a contact face formed of a material selected to minimize damage to the film and enhance gripping of the film. With movement of the chain, then, the pads of the opposing conveyors are sequentially brought into contact with, and thus transport, the film. Due to the viscoelastic property of many films subject to stretching, it is not necessary to have a continuously gripped interface with the film along the take-away device, such that the intermittent interface provided by the discrete pad tracks is highly viable. While the pliant or soft contact face of each pad may not impart overt defects into the film, for many end-use applications, an even minor alteration of the film is unacceptable and oftentimes the region of the film at which interface with the pads occurs must be discarded. In recognition of this process restriction, manufacturers endeavor to minimize the extent to which any defect imparted by the pads propagates across a width of the film by, for example, selecting a soft or pliant material. When handling highly thin films (e.g., on the order of 17 microns or less), however, the likelihood of imparting unacceptably large or problematic imperfections increases substantially regardless of the material utilized for the pad contact face, especially at locations between neighboring pads.
In light of the above, a need exists for film stretching apparatuses having a take-away device configured to interface or grip film in a manner that induces minimal defects.